Effect of Dose and Fasting on Oral Sugar Test Responses in Insulin Dysregulated Horses

The oral sugar test (OST) is frequently used to identify insulin dysregulated (ID) equines. The effect of fasting and varying sugar dose for the OST has been investigated in the pony but little work has been done in the horse. This study aimed to investigate (1) an OST response with access to forage continued until the time of the OST or prevented for 3 hours prior to the OST and (2) responses of ID and non-insulin dysregulated (NID) horses to two different OST doses. Twenty-one mixed-breed horses (14.8 ± 3.2 years; 574.3 ± 83.3 kg) were used in two randomized crossover studies. Seven ID and seven NID horses were used in study A, and eight ID and eight NID in study B. Study A horses underwent an OST (0.15 mL/kg BW) either after a fast (FA) or directly off pasture (FE). Study B horses received either a low (LD; 0.15 mL/kg BW) or high dose (HD; 0.45 mL/kg BW) OST on one occasion each. Blood was collected at basal (T0), and post-60 minute (T60) for later determination of insulin (RIA). Data were analyzed via ANOVA with repeated measures. ID horses had significantly (P < 0.05) greater insulin responses than NID for all OSTs. There was no statistical difference between LD versus HD mean insulin concentrations (T0, T60, delta insulin) for either ID or NID horses. ID had higher T0 (P < 0.01) for FE compared to FA; however, FE and FA did not significantly affect T60 or delta insulins (DI) concentrations.